fix(TPC): maxBitratesVideo break screenshare

In Chrome, if 'maxBitratesVideo' options are specified
and 'capScreenshareBitrate' is disabled, then simulcast is enabled
for screenshare. Chrome in plan B, does not send screen stream if
'maxBitrate' are set and simulcast is enabled.

modules/RTC/TraceablePeerConnection.js

     }
 
     const videoType = localVideoTrack.videoType;
+    const planBScreenSharing = browser.usesPlanB() && videoType === VideoType.DESKTOP;
 
     // Apply the maxbitrates on the video track when one of the conditions is met.
     // 1. Max. bitrates for video are specified through videoQuality settings in config.js
     // 2. Track is a desktop track and bitrate is capped using capScreenshareBitrate option in plan-b mode.
     // 3. The client is running in Unified plan mode.
     if (!((this.options.videoQuality && this.options.videoQuality.maxBitratesVideo)
-        || (browser.usesPlanB() && this.options.capScreenshareBitrate && videoType === VideoType.DESKTOP)
+        || (planBScreenSharing && this.options.capScreenshareBitrate)
         || browser.usesUnifiedPlan())) {
         return Promise.resolve();
     }
     if (this.isSimulcastOn()) {
         for (const encoding in parameters.encodings) {
             if (parameters.encodings.hasOwnProperty(encoding)) {
-                // On chromium, set a max bitrate of 500 Kbps for screenshare when
-                // capScreenshareBitrate is enabled through config.js and presenter
-                // is not turned on.
-                const bitrate = browser.usesPlanB()
-                    && videoType === VideoType.DESKTOP
-                    && this.options.capScreenshareBitrate
-                    ? presenterEnabled ? this.videoBitrates.high : DESKSTOP_SHARE_RATE
-                    : this.tpcUtils.localStreamEncodingsConfig[encoding].maxBitrate;
+                let bitrate;
+
+                if (planBScreenSharing) {
+                    // On chromium, set a max bitrate of 500 Kbps for screenshare when capScreenshareBitrate
+                    // is enabled through config.js and presenter is not turned on.
+                    // FIXME the top 'isSimulcastOn' condition is confusing for screensharing, because
+                    // if capScreenshareBitrate option is enabled then the simulcast is turned off
+                    bitrate = this.options.capScreenshareBitrate
+                        ? presenterEnabled ? this.videoBitrates.high : DESKSTOP_SHARE_RATE
+
+                        // Remove the bitrate config if not capScreenshareBitrate:
+                        // When switching from camera to desktop and videoQuality.maxBitratesVideo were set,
+                        // then the 'maxBitrate' setting must be cleared, because if simulcast is enabled for screen
+                        // and maxBitrates are set then Chrome will not send the screen stream (plan B).
+                        : undefined;
+                } else {
+                    bitrate = this.tpcUtils.localStreamEncodingsConfig[encoding].maxBitrate;
+                }
 
                 logger.info(`${this} Setting a max bitrate of ${bitrate} bps on layer `
                     + `${this.tpcUtils.localStreamEncodingsConfig[encoding].rid}`);